148 



REPTILES AND BIRDS. 



or Marsh Tortoises known, which are spread over all parts of the 

 globe, but principally in warm and temperate regions. Such are the 

 Cistudo, Emydes, and Trionyx. 



The Elodians have none of the sluggishness of the Land 

 Tortoises ; they swim with facility, and on land they walk with 

 considerable rapidity. Their eggs are white, and nearly spherical, 

 with a calcareous shell, and are deposited in a hollow dug in the 

 soil or sand, as the Land Tortoises' are, the place chosen being 



Fig. 34. Mud Tortoise. 



generally situated on the banks of some secluded stream ; the 

 number of eggs increasing as the animal approaches maturity. 



The Elodians are divided into Cryptodcres and Plcurodcres : the 

 former distinguished from the latter by the retractile power they 

 possess of concealing their cylindrical neck, with its sheath of loose 

 skin, under the middle of the carapace ; the head, the width of 

 which is nearly equal to the height at the occiput ; the eyes always 

 lateral, and their orbit so large that the diameter of the cavity 

 nearly equals a fourth of the whole cranium ; and the jaws, which are 

 strong, sometimes trenchant, in others are dentated on the edge. 

 In the larger number of species the anterior extremity of the upper 



